At site A Figure 4 , the drainage is offset by 60— m, depending on the chosen projection of upstream piercing line incised channel or northern valley slope , whereas the southern slope of Guano valley site B; Figure 4 is displaced between 60 and 90 m depending on the projection of used piercing line, i.
Assuming that the drainage has started to incise after the avalanche deposits emplacement, this provides a slip rate range value between 1 and 2.
The flat upper surface of this lava flow is clearly bumped along a tiny linear ridge in continuation of the fault. However, the lateral offset of the lava flow edge cannot be inferred from the DEM analysis. Note that other oblique strand has been detected by Baize et al. Summing up these two segments, a value of 3. Although we do not know the age of this incision feature, we suggest that it could have formed after the retreat of glaciers that filled the valley down to — m asl.
Consequently, the minimum slip rate there may then range between 2. In Rumipamba, Winter et al. We provide an updated analysis of slip rate 2. Note that, accounting for the The Huisla segment is poorly documented. The only indication is a 60 m deflection of a drainage incision in the Huisla avalanche deposits, overlooked with DEM and checked in the field.
The only temporal constrain available is the — ka age of the Huisla-related avalanche deposits Bablon et al. The drainage deflection is located on the same lineament as the recent rupture visible in the road cut 5 km to the north-east Figure 8B. In Pisayambo area, we could map in different places the horizontal offset of moraines, between 15 and 20 meters. These values would end up with a relatively low slip rate ranging between 0. Concerning the NS compressive structures, the mean Plio-Quaternary shortening rate across the IAV has been previously estimated around 1.
Assuming that those border faults are reverse and that the slip vector is for both faults parallel to the LOS, we can easily estimate their geodetic slip rates. When projecting the LOS onto the two fault planes, we end up with actual slip rates of 1. The lower and upper bound values of slip rate are summarized in Table 1. Table 1. Summary of slip rate along the 90 km section of the Pallatanga fault, based on observed offsets of landscape features and stratigraphic layers. We recompiled the original radiocarbon ages obtained in Baize et al.
We provide details of this analysis in Supplementary Material. The geological interpretation of the trench T2 Figure 12A , located a few tens of meters south of the trench visible on Figure 2 , led to propose a series of 4 paleoearthquakes, i. We supposed this last event to be the Riobamba earthquake, which seems to be the best candidate for this rupture, because of the vicinity of its epicentral location.
The perpendicular-to-fault trench only gives access to the vertical component of late Holocene slip. However, Winter et al. Using those values, the individual net slip during E1—E4 events range between 2 and 8. Figure 12B illustrates the slip versus time history that can be inferred from the trench result.
Over the 3 earthquake cycles, the cumulative fault displacement is between 7. This strongly suggests that this soil was developed promptly after a morphogenic earthquake that created space for sediment accumulation. Illustration of the slip versus time history inferred from the Rumipamba trench results. A Geological section of trench T2 dug in in Rumipamba Baize et al.
We simplified the drawing in representing in a same caption the bedrock fragment-rich horizons colluvial wedges , the soil units and the bedrock, as well as the fault planes and surrounding shattered rocks.
We underline the stratigraphic location of the event horizons that are used in the OxCal analysis. B X -axis is calibrated date; black bell-shaped curves are probability density functions of earthquake dates resulting from the stratigraphic model and the resulting Bayesian estimation of dates determined by OxCal online tool. Gray shaded boxes depict the uncertainty ranges of paleoearthquakes including age uncertainty from OxCal model and slip range from possible slip vector range.
A value of paleoseismological slip rate is extracted from this curve, depending on the initial slip vector hypothesis. The cumulative vertical component is around 4 meters Figure 4. The hand-made trench, as presented in the Supporting information of Champenois et al. This section, as well as the general geomorphology see picture , in Supplementary Material , suggest the occurrence of two surface-rupturing events by the alignments of pebbles within the soil, one between and cal B.
C, and the other between and cal B. However, the sedimentary signal is too weak to go further and estimate vertical separation, for instance. We estimate that multiple-segment ruptures could occur along the Pallatanga fault system. Biasi and Wesnousky , , based on a worldwide historical database, propose that the likelihood of rupture propagation is linearly related to the width of surface steps and gaps in strike-slip faults. However, recent events suggest that those rules can be significantly overstepped, such as the M7.
During this same event, some sections of the Canterbury surface ruptures intersect at orthogonal angle Litchfield et al. Such an event could also mobilize the nearby reverse segment in the IAV. Historical examples Denali, Alaska; Kaikoura, New Zealand have shown that a combination of strike-slip and thrust segments can rupture in one earthquake. Unfortunately, we do not have sufficiently continuous and reliable data to validate this statement based on the most recent earthquake surface rupture two sites distant of 40 km, from Rumipamba to Igualata , or based on paleoseismological recordings one trenched site in Rumipamba and one outcrop in Sicalpa at 10 km, see Baize et al.
Keeping in mind the whole range of uncertainty and assuming a constant earthquake rate over time, the slip versus time distribution at the Rumipamba site suggests that the fault behavior is not perfectly periodic, but is better matched by a slip predictable model Figure The earthquake E1 triggered too soon in a time predictable model.
Additional data are needed to constrain this first appreciation, to understand if this is a common feature all along the Pallatanga fault.
One first-choice site to explore is the Igualata summit, and another one is close to the pass between the Rio Pangor valley and the Cajabamba and Laguna Colta site , see Supplementary Material. In terms of slip rate, our new data fill a gap of knowledge in the middle of the CCPP fault system between the south-westernmost CCPP section the fault system crosses the littoral plain of Puna Island with minimal Holocene slip rate of 6—8 mm year Dumont et al.
In Figure 13 and Table 1 , we report the whole set of slip rate values estimated from both our field data and available datings or stratigraphic information. We ranked the values of slip rates obtained in the previous section with respect to their reliability, roughly estimated based on the robustness of the age of displaced markers and the precision or accuracy of the piercing lines. We consider the slip rate reliable when the age is estimated for a dated marker 14 C or K-Ar and when the piercing line is geometrically well-constrained.
Speculative clues are those for which none of those conditions are satisfied. In between, questionable cases bear rather reliable age or offset. Based on this, there could be an increase of slip rate around the Igualata area, north of Rumipamba where the slip rate estimate is reliable, and south of Pisayambo. This trend is mainly driven by two spots with questionable results both due to poorly constrained ages of displaced deposits; those rates could be revised with further data acquisition. Despite slip partitioning from the Pallatanga — Rumipamba segments to the Pisayambo fault zone, on one hand, and to the north-south folds on the other hand, there seems to be a variability of slip rate along strike.
Spatial changes are described both along long-living and segmented faults, like the km-long Doruneh Fault in Iran Farbod et al. In this latter case, variability is distributed over comparable along-strike distances. This is similar to the Pallatanga fault system. Along the Eastern California Shear Zone, the along-strike variation of strain is interpreted as the consequence of transfer to distributed structures off the fault of concern. This then suggests that this kind of variability could be a common trend of young and evolving fault systems, such as the Pallatanga Fault.
Note that back and forth deformation transfer and interaction between faults is also evoked to explain temporal changes at a single site Gold et al. Synthesis of slip rate estimation along the section of the 90 km Pallatanga fault system of concern. This graph is based on observed offsets of landscape features and stratigraphic layers, and related uncertainties and reliability, reported in Table 1. The fault map, when viewed at the regional scale, suggests that there are changing patterns from SW Pallatanga area to NE Riobamba then Pelileo areas with multiplication of fault sections, branches, and parallel strands.
Long-living crustal magmatic bodies are known to make the lateral propagation of faults more difficult because they modify the thermo-mechanical properties of the bedrock Dumont et al. It has also been proposed that the presence of a volcano could preclude the rupture propagation during a single earthquake Yagi et al.
Growth of the north-south trending active folds in the IAV is poorly constrained in time, though their onset is probably not older than several hundred of thousand years like the magmatic bodies intersecting the PF according to their immature morphology Alvarado, , and those features could be seen as the product of this interaction. This hypothesis will be tested further.
The primary objective of this work is to contribute to seismic hazard analyses for a densely inhabited area of central Ecuador. To achieve this aim, we provide a detailed description of available and newly acquired data on one major and emblematic fault zone, the Pallatanga fault, and its related structures. This compilation is a first step in building realistic fault models for future Probabilistic Seismic Hazard Analyses in Ecuador, providing updated and comprehensive fault maps and related earthquake geology parameters, such as slip rates, timing and magnitude of past earthquakes and their uncertainties.
We are aware of the heterogeneity of the data and further studies will have both 1 to enrich the existing fault database in terms of geology mapping, trenching, drilling, dating , geophysics sub-surface profiling , geodesy GNSS, InSAR and seismology near-fault monitoring, relocating clusters information, and 2 to expand this endeavor to other parts of the same seismotectonic province. To date, the scarcity of earthquake geology data in Ecuador clearly preclude developing a segmentation model of the Pallatanga fault system like along well-studied fault systems, such as the Wasatch Fault DuRoss et al.
Yet, recent large earthquakes illustrate that multiple segment ruptures are not rare and that unexpected scenarios with either kinematical or geometrical complex configurations M7 Darfield: Atzori et al. To test those scenarios in Ecuador, one could use concepts and tools recently developed for Probabilistic Seismic Hazard Analysis that consider potential complexities. For instance, Chartier et al. Then, one can relax the rupture segmentation and calculate the rate of multiple segment ruptures based on geological constrain in consistency with seismological datasets.
This approach is a relevant way to explore the behavior of the faults of concern. We suggest that it would be worth to develop similar earthquake geology approaches across the continent of South America, in parallel to geophysical data acquisition. The instrumental and historical record of earthquakes covering several hundred years at best and the geodetic measurements, fail to capture alone the behavior of crustal faults, to describe their complete earthquake productivity rate and the long recurrence of large events.
Those geological topics are critical knowledge for developing fault models and related seismic hazard analyses, beside seismology and geodesy, whatever the targets national scale maps or site-specific studies. The dataset generated during the study is compiled in Supplementary Material as a catalog of localized observations. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Alvarado, A. Google Scholar. Partitioning of oblique convergence in the Northern Andes subduction zone: migration history and the present-day boundary of the North Andean Sliver in Ecuador. Tectonics 25, — Active tectonics in quito, Ecuador, assessed by geomorphological studies, GPS data, and crustal seismicity.
Tectonics 33, 67— Angster, S. Late quaternary slip rates for faults of the central walker Lane Nevada, USA : spatiotemporal strain release in a strike-slip fault system. Geosphere 15, — Araujo, S.
Aspden, J. The geology and Mesozoic collisional history of the cordillera real. Tectonophysics , — Atzori, S. Solid Earth B Bablon, M. Interactions between volcanism and geodynamics in the southern termination of the Ecuadorian arc. Tectonophysics , 54— Baize, S. Paleoseismology and tectonic geomorphology of the Pallatanga fault Central Ecuador , a major structure of the South-American crust.
Geomorphology 23, 14— Barberi, F. Coca-Codo Sinclair Project , Quito , Beauval, C. A new seismic hazard model for Ecuador. Locations and magnitudes of historical earthquakes in the sierra of Ecuador An earthquake catalog for seismic hazard assessment in Ecuador. Bernard, B. Volcanes cuaternarios del Ecuador continental.
ReasearchGate doi: Biasi, G. Steps and gaps in ground ruptures: empirical bounds on rupture propagation. Bends and ends of surface ruptures. Bronk Ramsey, C. Bayesian analysis of radiocarbon dates.
Radiocarbon 51, — Champenois, J. Evidences of surface rupture associated with a low-magnitude M w 5. Solid Earth. Large-scale inflation of tungurahua volcano Ecuador revealed by persistent scatterers SAR interferometry. Chartier, T. Earthquake in Ecuador: A deep earthquake with a magnitude of 7. Magnitude 6. A magnitude 6. Two strong earthquakes shook the northwestern coastal area of Ecuador on Sunday night, the latest aftershocks from a powerful tremor that killed hundreds of people in April.
Authorities said there were no immediate reports of damage or injuries from the latest quakes. Ecuador Shaken By 7. A powerful 7. Nocquet, P. Jarrin, M. Mothes, R. Grandin, F. Rolandone, B. Delouis, H.
Yepes, Y. Font, D. Fuentes, M. Laurendeau, D. Cisneros, S. Hernandez, A. Sladen, J. Singaucho, H.
0コメント